Facet optical coupler

ABSTRACT

Techniques for forming a facet optical coupler to couple light at an edge of silicon substrate are described. The facet optical coupler includes a silicon substrate, a layer of second material disposed on the silicon substrate and in direct contact with the edge of the silicon substrate, and an undercut region disposed between a portion of the silicon substrate and the layer of second material. The undercut region is offset from the edge to provide mechanical integrity of the facet optical coupler to improve production of photonic integrated circuits having the facet optical coupler from a wafer.

RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §120 of U.S.application Ser. No. 14/796,678, filed on Jul. 10, 2015 under AttorneyDocket No. A1117.70007US01 and entitled “FACET OPTICAL COUPLER,” whichis hereby incorporated herein by reference in its entirety. ApplicationSer. No. 14/796,678 claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application Ser. No. 62/023,529 entitled “FACETOPTICAL COUPLER,” filed Jul. 11, 2014, which is incorporated herein byreference in its entirety.

BACKGROUND Field

The present application relates to a facet optical coupler and relatedapparatus and methods.

Related Art

A photonic integrated circuit (PIC) includes integrated opticalcomponents on a substrate. A facet optical coupler couples an externaloptical component, such as an optical fiber, to a waveguide of the PICvia an edge of the substrate.

BRIEF SUMMARY

According to an aspect of the present application a facet opticalcoupler is provided. The facet optical coupler comprises a siliconsubstrate having an edge, a waveguide, a layer of second material, andan undercut region. The layer of second material is disposed on thesilicon substrate and in contact with the silicon substrate at the edge.The waveguide is embedded within the layer of second material. Theundercut region is disposed between a portion of the silicon substrateand the layer of second material.

According to another aspect of the present application a method ofmanufacturing a facet optical coupler is provided. The method comprisesforming, in a surface of a silicon substrate having a first index ofrefraction, a region having a second index of refraction less than thefirst index of refraction. The silicon substrate further comprises anedge. The method further comprises forming a layer of second material onthe surface of the silicon substrate such that an edge of the layer ofsecond material is substantially co-planar with the edge of the siliconsubstrate and forming a waveguide embedded in the layer of secondmaterial.

BRIEF DESCRIPTION OF DRAWINGS

Various aspects and embodiments of the application will be describedwith reference to the following figures. It should be appreciated thatthe figures are not necessarily drawn to scale. Items appearing inmultiple figures are indicated by the same reference number in all thefigures in which they appear.

FIGS. 1A and 1B are cross-sectional diagrams of examples of facetcouplers.

FIG. 2 is a cross-sectional diagram of a facet coupler having an offsetundercut region, according to a non-limiting embodiment.

FIGS. 3A-3C illustrate, in cross-sectional views, alternative examplesof the facet coupler of FIG. 2 taken along line A-A′ in FIG. 2.

FIGS. 4A-4C illustrate, in cross-sectional views, alternate examples ofthe facet coupler of FIG. 2 taken along line B-B′ in FIG. 2.

FIG. 5 is a plot of power loss as a function of incident angle forpolarizations TE and TM in a facet coupler of the type illustrated inFIG. 2.

FIG. 6 is a cross-sectional diagram of an example of a facet coupleraccording to a non-limiting embodiment.

FIG. 7 is a cross-sectional diagram of an example of a facet coupleraccording to a non-limiting embodiment.

FIG. 8 is a schematic illustrating an exemplary method for forming afacet coupler according to a non-limiting embodiment.

DETAILED DESCRIPTION

Aspects of the present application relate to a facet optical coupler ofa silicon-based integrated device. The contrast in the indices ofrefraction between silicon and other materials (e.g., silicon dioxide,air) allows the formation of silicon-based photonic integrated circuits(PICs) with submicron waveguide dimensions. While PICs can includemultiple components because of these submicron dimensions, coupling anoptical fiber as an optical input or output to a submicron waveguide onthe PIC can be challenging due to the mismatch in mode field sizebetween the optical fiber and the waveguide. For example, a standardsingle-mode optical fiber has a mode field diameter of approximately 10microns, while a silicon submicron waveguide has a mode field diameterof less than 1 micron. Direct coupling or butt coupling between thefiber and the waveguide may result in significant coupling loss, such asmore than 20 dB in some instances. Thus, coupling techniques thataccount for the significantly different mode sizes between an opticalfiber and a submicron silicon waveguide may improve performance ofsilicon based PICs by reducing coupling loss.

One type of coupling technique may include a grating coupler on anintegrated device, where mode matching can be achieved by forming thedimensions of the waveguide and positioning additional elements todirect scattered light. Such couplers may have a narrow opticalbandwidth because the capability to direct light between the fiber andthe waveguide depends on the extent to which phase matching conditionsare met. Another type of coupling technique may vary the dimensionsand/or shape of the waveguide to improve mode matching by expanding themode field of the waveguide in one or more dimensions to sufficientlycouple with the mode field of the optical fiber. The waveguide may betapered in one or more dimensions perpendicular to the direction oflight propagation within the waveguide. However, challenges may arise inimplementing these designs because of the index of refraction of thesurrounding materials, including cladding materials and the siliconsubstrate. The large contrast in the indices of refraction between thesilicon and cladding materials provides that the tapered waveguide has anarrow tip, such as 50 nm or less, to sufficiently expand the modefield. The silicon substrate may be located within 3 microns or less ofthe waveguide and may interfere with the mode field expansion of thewaveguide due to this proximity.

Applicants have appreciated that incorporating a layer of secondmaterial in a facet optical coupler of a photonic integrated circuit(PIC) may improve the coupling of light between an optical fiber and thePIC. The layer of second material may function as an auxiliary waveguidebridging the external coupling beam and the submicron silicon waveguide.Accordingly, aspects of the present application relate to a facetoptical coupler with a layer of second material disposed on a siliconsubstrate and in direct contact with the silicon substrate. A waveguideof the integrated device may be embedded within the layer of secondmaterial. In some embodiments, the layer of second material and thesilicon substrate may have coplanar edges.

A cross-sectional view of a facet optical coupler is shown in FIG. 1A.Facet optical coupler 100 includes silicon substrate 102, layer 104,silicon waveguide 106, and overlay region 108. The overlay region mayact as a waveguide and may have a refractive index higher than that oflayer 104. Overlay region 108 may define the size and shape of the modefield. Overlay region 108 may include a taper, and the buried waveguide106 may include a taper as well, both of which are to direct lightcoupled into overlay region 108 towards waveguide 106, as shown by thedashed lines in FIG. 1A. The dimensions of overlay region 108 at theedge of the facet optical coupler (at the arrow shown in FIG. 1A) mayconstrain expansion of the mode and may lack suitable coupling for someoptical fibers, such as optical fibers with a large mode field size(e.g., approximately 10 microns).

A cross-sectional view of another facet optical coupler is shown in FIG.1B. Facet optical coupler 110 includes silicon substrate 112, undercutregion 120, layer 114, waveguide 116, and cladding 118. Undercut region120 separates silicon substrate 112 from waveguide 116 near the facet,reducing interference in the mode field expansion during coupling offacet optical coupler 110 with an optical fiber. As shown by the dashedlines in FIG. 1B, the mode field expands near the edge of the integrateddevice and contracts in a direction of light propagation along waveguide116. Undercut region 120 may have a lower refractive index than that ofboth the layer 114 and silicon substrate 112.

In some cases, undercut region 120 may be air, which results in layer ofsecond material 114 being suspended over silicon substrate 112 at theedge. Having undercut region 120 located at an edge where layer 114 issuspended reduces the mechanical integrity of facet optical coupler 110.This lack of structural integrity may be particularly problematic duringformation of an integrated device having facet coupler 100 because thestructure of the facet coupler may break or become faulty when a waferis diced along edges of the integrated device and/or when adheringoptical fibers to the edges of the device for packaging of the device.

Applicants have appreciated that offsetting the undercut region from anedge of a silicon substrate can result in suitable coupling performanceeven if there is a region where the silicon substrate forms an interfacewith a layer of second material. Accordingly, aspects of the presentapplication relate to forming a facet optical coupler with an undercutregion offset a distance from an edge of an integrated device. Such anundercut region may improve the mechanical integrity of the device byadhering a silicon substrate and a layer of second material at an edgeof the integrated device. The distance by which the undercut region isoffset from the edge may be selected to provide suitable couplingefficiency for the facet coupler while maintaining sufficient mechanicalintegrity. In this manner, a facet optical coupler having an offsetundercut region may facilitate production and processing of integrateddevices having the facet optical coupler while providing desiredcoupling efficiency with an external optical component.

In some embodiments, the undercut region may be formed by removing aportion of the silicon substrate, and in some embodiments, filling theremoved portion with a filler material having a lower index ofrefraction than the silicon substrate and the second material.

The aspects and embodiments described above, as well as additionalaspects and embodiments, are described further below. These aspectsand/or embodiments may be used individually, all together, or in anycombination of two or more, as the application is not limited in thisrespect.

An exemplary facet optical coupler having an offset undercut region isshown in FIG. 2. The facet optical coupler is configured to couple lightbetween an optical fiber positioned proximate to edge 214 and waveguide208. As an example, the arrow and dashed curved lines shown in FIG. 2indicate the propagation of light through the facet optical coupler 200from an optical fiber near edge 214 into waveguide 208 along thez-direction, where the spread of the curved dashed lines indicates avariation in mode field size. Facet optical coupler 200 may be used asan optical input or an optical output by changing the direction of lightpropagation. Waveguide 208 may include silicon and may be considered asilicon waveguide. Although FIG. 2 shows the propagation of input light,facet coupler 200 may be configured as an optical output where lightpropagating through waveguide 208 couples to an optical fiber or otherexternal optical component positioned proximate to edge 214.

Facet optical coupler 200 includes silicon substrate 202 having an edge212. A layer of second material 206 is disposed on silicon substrate202. Waveguide 208 may be embedded within layer of second material 206.Layer of second material 206 may be in direct contact with siliconsubstrate 202 at edge 212. The surface of silicon substrate 202 andlayer of second material 206 may form an interface, where an edge of theinterface is located at edge 212 of silicon substrate 202. The interfacebetween silicon substrate 202 and layer of second material 206 may besubstantially perpendicular to edge 212 of silicon substrate 202. Insome embodiments, edge 214 of layer of second material 206 may becoplanar with edge 212. Layer of second material 206 may have a lowerindex of refraction than substrate 202 and may have higher index ofrefraction than cladding layer 210. Layer of second material 206 mayinclude a dielectric material, and may be considered a dielectric layer.Layer of second material 206 may include one or more of SiO₂, SiON, andSi₃N₄, by way of example and not limitation.

Facet optical coupler 200 also includes undercut region 204 disposedbetween a portion of silicon substrate 202 and layer of second material206. In some embodiments, D may be at least 10 microns, at least 20microns, at least 50 microns, at least 100 microns, or at least 500microns. Undercut region 204 may have an index of refraction lower thanthat of layer of second material 206. In embodiments where layer ofsecond material 206 includes silicon dioxide, undercut region 204 mayhave an index of refraction less than silicon dioxide. Undercut region206 may include a gas (e.g., air), a liquid (e.g., index-matching oil),and/or a solid filler material (e.g., thermal- or UV-curable adhesive orepoxy). Undercut region 204 may be offset from edge 212 in a directionperpendicular to edge 212 of silicon substrate 202. The offset directionof undercut region 204 from edge 212 may be in a direction of lightpropagation through facet optical coupler 200 such that there is aregion where silicon substrate 202 and layer of second material 206 arein contact between edges 212 and 214 and undercut region 204. Edges 212and 214 may form an edge of an integrated device having facet coupler200, and undercut region 204 may be offset by a certain distance fromthe edge of the device. By having undercut region 204 offset from edge212, layer of second material 206 contacts silicon substrate 202 at edge212 and provides mechanical integrity to the edge of the integrateddevice, rather than having a suspended structure at the edge, such asthe facet coupler structure shown in FIG. 1B. A facet optical coupler,such as 200, where an undercut region is offset from an edge of anintegrated device reduces mechanical fragility at the edge, allowing forimproved production and processing of the integrated device when dicinga wafer and/or attachment of one or more optical fibers to theintegrated device at the edge.

Facet optical coupler 200 may include cladding layer 210 positioned onlayer of second material 206. Cladding layer 210 may provide an index ofrefraction contrast at an interface between cladding layer 210 and layerof second material 206. Cladding layer 210 may have a lower index ofrefraction than layer of second material 206. Cladding layer 210 mayinclude air or, in embodiments where layer of second material 206 issilicon dioxide, may include a material with an index of refractionlower than silicon dioxide. In some embodiments, both cladding layer 210and undercut region 204 may include the same material.

Layer of second material 206 may have any suitable dimensions configuredto couple light between waveguide 208 and edge 214 such that lightcouples to an external optical component positioned proximate to edge214. Exemplary cross-sectional configurations of facet coupler 200 alongline A to A′, which is at edge 212 of silicon substrate 202 and at edge214 of layer of second material 206, are shown in FIGS. 3A, 3B, and 3C.Layer of second material 206 contacts silicon substrate 202, forming aninterface between layer 206 and substrate 202 along the y-direction. Across-sectional width of layer of second material 206 may vary in they-direction to include a portion having a larger width in contact withsilicon substrate 202, such as shown in FIGS. 3B and 3C. In someinstances, layer of second material 206 may have a width that covers asurface of substrate 202 such that cladding layer 210 lacks directcontact with substrate 202 as shown in FIG. 3C. A direction for thevariation in cross-sectional width of layer of second material 206 maybe considered perpendicular to a direction of light propagation andparallel to a surface of substrate 202.

Undercut region 204 may have any suitable configuration. In someembodiments, undercut region 204 may extend beyond a dimension of layerof second material 206. Exemplary cross-sectional configurations offacet optical coupler 200 along line B to B′ are shown in FIGS. 4A, 4B,and 4C, corresponding to FIGS. 3A, 3B, and 3C, respectively. As shown inFIG. 4A, undercut region 204 may have a cross-sectional width,D_(width), and height, D_(height). Undercut region 204 may have a largercross-sectional width than layer of second material 206 such thatundercut region 204 extends beyond layer of second material 206 alongthe y-direction, as shown in FIGS. 4A and 4B. In some embodiments,undercut region 204 may have a similar or the same cross-sectional widthas layer of second material 206, as shown in FIG. 4C. Thecross-sectional width of undercut region 204 may be a dimension of theundercut region 204 that lies in a direction parallel to an interfacebetween undercut region 204 and layer of second material 206, andperpendicular to a direction of light propagation. In some embodiments,the cross-sectional width of undercut region 204 may be at least 10microns or at least 14 microns. Undercut region 204 provides aseparation between substrate 202 and layer of second material 206 byhaving a cross-sectional height, such as D_(height) along thex-direction shown in FIG. 4A. The dimension of undercut region 204corresponding to a cross-sectional height may lie in a direction that isperpendicular to an interface of undercut region 204 and layer of secondmaterial 206 and perpendicular to a direction of light propagation. Insome embodiments, the cross-sectional height of undercut region 204 maybe at least 2 microns or at least 10 microns.

Some embodiments of the present application relate to layer of secondmaterial 206 having a lower refractive index than silicon substrate 202,where layer of second material 206 may act as a waveguide with someamount of optical power leakage into substrate 202. The leakage occursprimarily over a region that light propagates where there is aninterface between substrate 202 and layer of second material 206, suchas dimension D shown in FIG. 2. Within this region, as an optical beamhits the interface between substrate 202 and layer 206 there is acertain amount of optical loss that occurs from refraction at theinterface. In some embodiments, the design of layer of second material206 as a waveguide may reduce optical power leakage, such as byconfiguring layer 206 to reduce the amount of power loss per a hit ofthe optical beam at the interface. In some embodiments, reducing adimension of this region along a direction of light propagation, such asthe length of D, may reduce the refraction loss and may improve couplingefficiency of the facet optical coupler.

In some embodiments, the power leakage at the interface between layer ofsecond material 206 and substrate 202 may be reduced by suitableselection of the dimensions of the layer of second material 206 and/orthe refractive indices of layer of second material 206 and claddinglayer 210. The optical modes supported by layer of second material 206may have some amount of leakage when the refractive index of substrate202 is higher than that of layer of second material 206. The leakage canbe understood by way of a ray optics depiction of the propagation oflight through layer of second material 206. For each mode, light travelsat an angle from the propagation direction z, and is bounced back andforth at the interfaces of layer of second material 206. The amount ofpower leakage per unit length depends on the incidence angle at theinterface between layer of second material 206 and substrate 202. As theangle approaches 90 degrees from the normal of the interface, the lightbeam may nearly graze along the interface, and the power leakagedecreases. FIG. 5 is a plot illustrating the amount of leakage loss (onthe y-axis) as a function of incident angle for both transverse-electric(TE) and transverse-magnetic (TM) polarizations with the example of aninterface between silicon dioxide and silicon. At large incident anglesapproaching 90 degrees, the amount of leakage loss becomes small forboth TE and TM polarizations to provide sufficient coupling efficiencyfor facet coupler 200. One can obtain a large incident angle towards 90degrees by making the effective index of the mode very close to therefractive index of layer of second material 206. This in turn can beachieved by making the cross-sectional dimensions of layer of secondmaterial 206 relatively large and the index of the cladding 210relatively close to that of the layer of second material 206, whilepreserving a mode size comparable to that of the external coupling mode.

Dimensions of the layer of second material, the undercut region, and/orwaveguide may provide varying mode field size capabilities to allowsufficient coupling of an optical fiber, or other external opticalcomponent, with a desired mode field size to facet optical coupler. Someembodiments relate to configuring a layer of second material to reducepower lost through leakage, such as by tapering one or more dimensionsof the layer to alter the field mode within the layer of secondmaterial. The layer of second material may have a decrease in size takenin a direction moving from an edge of a silicon substrate to a waveguideby having a reduced dimension in the x-direction and/or y-direction.Some embodiments may include variation along the x-direction, such as avertical tapering and/or step profile of the height of the layer ofsecond material. An exemplary facet optical coupler is shown in FIG. 6where layer of second material 606 decreases in size from edge 612 ofsilicon substrate 602 to waveguide 608. As shown in FIG. 5, layer ofsecond material 606 may include a step profile, where a height of layer606 along the x-direction decreases by a certain amount at a locationoverlapping with undercut region 604 and non-overlapping with waveguide608. Cladding layer 610 positioned on layer of second material 606 mayhave one or more dimensions conforming to the variation of layer 606.Having a larger cross-sectional area of layer of second material 606 atedge 614 may facilitate mode overlap between an optical fiber positionedat edge 614 and facet coupler 600. By reducing the cross-sectional areaof layer 606, the mode field within layer 606 is sufficiently reduced toallow coupling of light to waveguide 608. Such a configuration mayimprove coupling efficiency between layer of second material 606 andwaveguide 608 and/or improve the fabrication process by relaxingfabrication requirements for the waveguide 608, such as requirementsrelated to including an inverse taper for waveguide 608.

Some embodiments relate to configuring a waveguide to have variation inone or more dimensions perpendicular to a direction of lightpropagation. A cross-sectional area of a waveguide may increase in adirection moving away from an edge the waveguide towards a centerportion of the waveguide. The waveguide may have a first portion thatoverlaps with an undercut region and a second portion that does notoverlap with the undercut region. The waveguide may increase in sizemoving in a direction from the first portion to the second portion. Anexemplary facet optical coupler is shown in FIG. 7 where waveguide 708has a variable dimension along the x-direction. As shown in FIG. 7,waveguide 708 may include a vertical step profile such that waveguide708 has a smaller dimension in a region where waveguide 708 and layer ofsecond material 706 couple. Facet optical coupler 700 also includessubstrate 702, layer of second material 706, and cladding layer 710.Such a configuration may improve coupling efficiency between layer ofsecond material 706 and waveguide 708 and/or improve the fabricationprocess by relaxing fabrication requirements for waveguide 708.

Some embodiments of the present application relate to a method ofmanufacturing a facet optical coupler having a structure describedherein. Any fabrication techniques suitable for silicon-based photonicintegrated circuits may be used according to some embodiments. FIG. 8shows steps of an example of a method of manufacturing a facet opticalcoupler, although other suitable methods and/or additional steps may beused to form any of the facet optical couplers described above. Method800 may start with a silicon substrate having a layer of second materialformed on the silicon substrate and a layer of silicon formed on thelayer of second material. In some embodiments, a wafer may include asilicon substrate, a layer of second material on the silicon substrate,and a layer of silicon on the layer of second material, such as asilicon-on-insulator (SOI) wafer. In other embodiments, method 800 mayinclude forming a layer of second material on a silicon substrate andforming a layer of silicon on the layer of second material.

Regardless of the techniques used to achieve this initial structure,method 800 may proceed by patterning a waveguide by act 810. Patterningthe waveguide may include removing a portion of the silicon layer toachieve a desired structure for the waveguide. Method 800 may includeforming additional second material on the patterned waveguide to form alayer of second material by act 820. In this manner, the waveguide maybe embedded in the layer of second material. This act may includeforming a layer of second material on a surface of the silicon substratesuch that an edge of the layer of second material is substantiallycoplanar with the edge of the silicon substrate. Method 800 may alsoinclude patterning the layer of second material by act 830 with asuitable structure and dimensions, such as the configurations shown inFIGS. 3A-3C. Some embodiments may include forming the layer of secondmaterial with a decreasing size taken in a direction moving from theedge of the silicon substrate to the waveguide.

Method 800 may include forming an undercut region in the siliconsubstrate at act 840. The undercut region may have an index ofrefraction that differs from the silicon substrate. The siliconsubstrate may have a first index of refraction and the undercut regionmay have a second index of refraction less than the first index ofrefraction. The undercut region having a second index of refraction maybe formed by removing a portion of the silicon substrate. For example,the portion of the silicon substrate may be removed by etching thesilicon substrate at the sides of the layer of second material, such asby isotropic etching. Such techniques may allow removal of silicon bothlaterally and vertically to form an undercut region positioned betweenthe silicon substrate and the layer of second material. In someembodiments, a protection layer (e.g., a photoresist mask) may be formedon the layer of second material to protect the layer of second materialwhile forming the undercut region. In some embodiments, patterning ofthe layer of second material and forming the undercut region may beperformed in a common set of steps by forming the protection layer in adesired pattern for the layer of second material and removing both thelayer of second material and the portion of the silicon substrateremoved to form the undercut region.

Forming the undercut region having the second index of refraction mayinclude positioning the undercut region at an offset from an edge of thesilicon substrate. In some embodiments, forming the undercut regionhaving the second index of refraction may include forming the undercutregion to extend beyond the layer of second material in a directionperpendicular to a direction of light propagation and parallel to aninterface of the undercut region having the second index of refractionand the layer of second material (e.g., a direction along the y-axis inFIGS. 4A-4C). In some embodiments, a dimension (e.g., D_(width) shown inFIG. 4A) of the undercut region having the second index of refraction isat least 10 microns. The dimension, D_(width), may be considered as adimension of the undercut region along a direction that is perpendicularto a direction of light propagation and parallel to an interface of theundercut region having the second index of refraction (e.g., a directionalong the y-axis in FIGS. 4A-4C).

In some embodiments, the waveguide may have a variable dimension movingin a direction of the propagation of light. The waveguide may include afirst portion overlapping the undercut region having the second index ofrefraction and a second portion not overlapping the undercut regionwhere the waveguide increases in size moving in a direction from thefirst portion to the second portion.

In some embodiments, method 800 optionally includes filling the undercutregion with a filler material having the second index of refraction byact 850. Filling the undercut region may include filling the undercutregion with a liquid (e.g., an epoxy) and/or curing the liquid to form asolid. In some embodiments, act 850 may alternatively or additionalinvolve forming a cladding layer disposed on the layer of secondmaterial, wherein the cladding layer has a lower index of refractionthan the layer of second material. In some embodiments, filling theundercut region with a filler material and forming the cladding layermay be performed in a common set of steps. In such embodiments, theundercut region and the cladding layer may include a common material.

Having thus described several aspects and embodiments of the technologyof this application, it is to be appreciated that various alterations,modifications, and improvements will readily occur to those of ordinaryskill in the art. Such alterations, modifications, and improvements areintended to be within the spirit and scope of the technology describedin the application. It is, therefore, to be understood that theforegoing embodiments are presented by way of example only and that,within the scope of the appended claims and equivalents thereto,inventive embodiments may be practiced otherwise than as specificallydescribed. In addition, any combination of two or more features,systems, articles, materials, kits, and/or methods described herein, ifsuch features, systems, articles, materials, kits, and/or methods arenot mutually inconsistent, is included within the scope of the presentdisclosure.

Also, as described, some aspects may be embodied as one or more methods.The acts performed as part of the method may be ordered in any suitableway. Accordingly, embodiments may be constructed in which acts areperformed in an order different than illustrated, which may includeperforming some acts simultaneously, even though shown as sequentialacts in illustrative embodiments.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. The transitional phrases “consisting of” and “consisting essentiallyof” shall be closed or semi-closed transitional phrases, respectively.

What is claimed is:
 1. A facet optical coupler comprising: a substratehaving opposing first and second two-dimensional edges; an undercutregion in the substrate, the undercut region being proximate to andoffset from the first two-dimensional edge of the substrate and beingdistal from the second two-dimensional edge of the substrate; and awaveguide having an end over the undercut region, wherein the undercutregion is closer to the first two-dimensional edge than is the end ofthe waveguide.
 2. The facet optical coupler of claim 1, wherein thesubstrate further includes a surface, and the facet optical couplerfurther comprises a layer of second material disposed on the surface ofthe substrate, wherein the waveguide is embedded in the layer of secondmaterial.
 3. The facet optical coupler of claim 2, wherein the layer ofsecond material has a surface substantially co-planar with the firsttwo-dimensional edge.
 4. The facet optical coupler of claim 2, whereinthe undercut region extends beyond the layer of second material in adirection perpendicular to a direction of the waveguide.
 5. The facetoptical coupler of claim 2, wherein a dimension of the undercut regionthat is parallel to an interface of the undercut region and the layer ofsecond material is at least 10 microns.
 6. The facet optical coupler ofclaim 2, wherein a dimension of the undercut region that isperpendicular to an interface of the undercut region and the layer ofsecond material is at least 2 microns.
 7. The facet optical coupler ofclaim 2, wherein the layer of second material decreases in size in adirection moving from the first two-dimensional edge of the substrate tothe waveguide.
 8. The facet optical coupler of claim 2, wherein thesecond material is a dielectric material.
 9. The facet optical couplerof claim 2, wherein the second material includes one or more of SiO₂,SiON, and Si₃N₄.
 10. The facet optical coupler of claim 2, furthercomprising a cladding layer on the layer of second material.
 11. Thefacet optical coupler of claim 10, wherein each of the cladding layerand the undercut region has a lower refractive index than the secondmaterial.
 12. The facet optical coupler of claim 1, wherein the undercutregion is offset from the first two-dimensional edge of the substrate byat least 20 microns.
 13. The facet optical coupler of claim 1, whereinthe waveguide has a first portion overlapping the undercut region and asecond portion not overlapping the undercut region, and wherein thewaveguide increases in size moving in a direction from the first portionto the second portion.
 14. A method of manufacturing a facet opticalcoupler, the method comprising: forming a region in a substrate suchthat the region has a lower index of refraction than the substrate andis positioned proximate to but offset from a first planar edge of thesubstrate and distal a second planar edge of the substrate opposite thefirst planar edge; and forming a waveguide terminating over the regionsuch that a distance between the waveguide and the first planar edge isgreater than a distance between the region and the first planar edge.15. The method of claim 14, wherein forming the region comprisespositioning the region at an offset of at least 20 microns from thefirst planar edge of the substrate.
 16. The method of claim 14, whereinforming the region comprises removing and filling a portion of thesubstrate with a filler material having the lower index of refraction.17. The method of claim 14, further comprising forming a layer of secondmaterial over the substrate and forming a cladding layer on the layer ofsecond material, wherein the waveguide is embedded in the layer ofsecond material and the cladding layer has a lower index of refractionthan the layer of second material.
 18. The method of claim 17, whereinforming the region comprises forming the region to extend beyond thelayer of second material in a direction perpendicular to a direction ofthe waveguide.
 19. The method of claim 17, wherein forming the layer ofsecond material comprises forming the layer of second material with adecreasing size in a direction moving from the first planar edge of thesubstrate to the waveguide.
 20. The method of claim 14, wherein formingthe waveguide comprises forming the waveguide with a first portionoverlapping the region and a second portion not overlapping the region,and wherein the waveguide increases in size moving in a direction fromthe first portion to the second portion.